* List<A extends String>}, we need to compare that {@code <A extends B>} is unequal to {@code <A
   * extends String>} in order to decide to use the transformed type instead of the original type.
   */
  static final class TypeVariableKey {
    private final TypeVariable<?> var;

    TypeVariableKey(TypeVariable<?> var) {
      this.var = checkNotNull(var);
    }

   * List<A extends String>}, we need to compare that {@code <A extends B>} is unequal to {@code <A
   * extends String>} in order to decide to use the transformed type instead of the original type.
   */
  static final class TypeVariableKey {
    private final TypeVariable<?> var;

    TypeVariableKey(TypeVariable<?> var) {
      this.var = checkNotNull(var);
    }